The present invention relates to a light source device of highly precise laser beam printers (LBP). More particularly, the invention relates to a light source device for a laser beam printer which can produce a stable output free from the occurrence generation of an overshooting (transient response) at a short-time pulse having a pulse width of not more than milliseconds for driving a laser diode and which can provide printing free from an irregularity in printed images.
Laser beam printers can print images by drawing an electrostatic image on a photosensitive drum with laser beams and depositing a toner (a powder ink) on a portion charged with a static electricity. In order to draw the image with this laser beam, a light source device using a laser diode is employed. With the application of a pulse current to the laser diode, the light source device is driven intermittently. In this case, as shown in FIG. 8, there is generally shown a characteristic such that a light output is gradually decreased after the electrification of the laser diode is started. There is a problem in that when a droop (xcex94P/Pxc3x97100%) which represents this lowering ratio is large, an irregularity in colors likely occurs. It can be considered that the cause of the irregularity in colors can be attributed to a rise in the temperature of the laser diode. As one method for decreasing this droop, the active layer is formed in a multiple quantum well structure (MQW) to suppress the heating of the laser diode (LD) chip itself, so that the threshold current Ith is decreased. However, when this MQW structure is adopted, the width of the energy distribution of carriers is narrowed down with the result that the oscillation spectrum width is narrowed down and a single mode oscillation is likely to be generated.
In the laser diode which oscillates in a single mode with the lowering of the above threshold current, the heating of the laser diode is reduced and the droop can be surely reduced. However, as FIGS. 9(a) through 9(b) show a driving pulse and a light output waveform, an overshooting (a transient phenomenon) A is generated at a rise portion of the pulse. The ratio of this overshooting A corresponds to the time of the order of 1 to 2 xcexcsec, but it extends over about 20% of a stable light output. On the other hand, recently, in order to improve the printing speed of the printer, the pulse width of this drive pulse is extremely narrowed down. When a request is made to narrow down the width to not more than milliseconds, particularly to micro seconds to sub-micro seconds, the influence of the above overshooting becomes a problem as the irregularity in colors in addition to the droop.
On the other hand, the laser diode oscillates in a multiple mode by forming the active layer into, for example, a bulk active layer, the threshold current rises as described above, so that LD chip is heated and a large droop is likely to be generated. At the same time, as shown in FIG. 9(c), the light output at the rise portion is lowered on the contrary with the result that the irregularity in colors is generated and a stable output of light cannot be obtained because of further self-excited oscillation within the pulse.
The present invention has been made in order to solve the above problem. An object of the invention is to provide a light source device for a laser beam in which a transient phenomenon (a transient response) at the time of the rise of the pulse is not generated, a stable light output is obtained, and images can be printed free from an irregularity in colors even in the case where a light source device for a laser beam printer is driven with a pulse which has a small pulse width at ON time such as not more than milliseconds, particularly micro-seconds to sub-micro seconds.
After a diligent investigation to remove the irregularity in colors in printed images caused by the transient phenomenon (transient response) at the rise portion of the pulse for driving the semiconductor laser in a short pulse width as described above, the inventors of the present invention have found that a relation is present between an oscillation spectrum of a laser, for example, Full Width at Half Maximum (FWHM) and a rate of change in the transient phenomenon (a ratio xcex94P/Poxc3x97100%, in which xcex94P is a difference (Ppxe2x88x92Po) between a light peak output Pp at a transience at the time of the rise and a stable output Po, refer to FIG. 9(b)). And the inventors of the present invention have found that a light source device for a laser beam printer can be provided, which device does not generate the irregularity in colors with a drive pulse on the order of milliseconds to micro seconds, by adjusting the above rate of change and by setting the rate to xc2x18% or less. The rate of change is changed by adjusting the light absorption quantity of the oscillation power, and the adjusting is performed by changing, for example, a stripe width, a composition of a current block layer, a distance between a current block layer and the active layer, a carrier concentration of a clad layer and a penetration degree of a light absorption layer (penetration place or the composition and the thickness of the absorption layer) into the current block layer.
According to the present invention, there is provided a light source device for a laser beam printer in which a semiconductor laser is driven with the pulse current having the minimum pulse width at the ON time on the order of milliseconds or less, the semiconductor laser comprising:
a double hetero junction structure in which an active layer is sandwiched with an n-type clad layer and a p-type clad layer; and
a current block layer having a stripe groove provided in one of the n-type and p-type clad layers;
wherein a rate of change on the order of micro seconds at a rise portion of a light output of the semiconductor laser at the time of being driven by the pulse current is set in xc2x18% or less, by adjusting at least one of a width of the stripe groove, the number of quantum wells of the active layer, a composition of the clad layers, a distance between the current block layer and the active layer, a composition of the current block layer, a carrier concentration of the clad layers and the formation of a light absorption layer into the current block layer.
Here, the rate of change refers to the ratio of a peak output at the rise portion by the pulse drive and the leveled (stabilized) light output which is represented in %. The fact that the rate of change is negative means that the light output at the rise time is smaller than the leveled light output.
With such a structure of the light source device for the laser beam diode, the influence of the transient phenomenon does not appear even when the drive pulse width for the semiconductor laser becomes short, so that a beautiful printed image free from the irregularity in colors can be provided.
According to another aspect of the present invention, there is provided a light source device for a laser beam printer in which a semiconductor laser is driven with a pulse current having a minimum pulse width at the ON time on the order of milliseconds or less, said semiconductor laser comprising:
a double hetero junction structure in which an active layer is sandwiched with an n-type clad layer and a p-type clad layer; and
a current block layer having a stripe groove provided in one of the n-type and p-type clad layers;
wherein the semiconductor laser is formed so that the semiconductor laser oscillates in a multiple mode in the vicinity of the threshold value of oscillation, and oscillates in a single mode at a current separated from the threshold value, by adjusting at least one of a width of the stripe groove, the number of quantum wells of the active layer, a composition of the clad layers, a distance between the current block layer and the active layer, a composition of the current block layer, a carrier concentration of the clad layers and the formation of a light absorption layer into the current block layer.
Here, the vicinity of the threshold value of oscillation refers to an operation current in the vicinity of the current value at which the laser begins to oscillate, and the current separated from the threshold value refers to an operation current larger than the threshold value, such as current of about 1.2 times of the threshold value current or more.
The active layer of the semiconductor laser is formed in the single or multiple quantum well (SQW or MQW) structure with the result that the rate of change can be reduced by the oscillation of the semiconductor laser in a multiple mode in the vicinity of the threshold value, during the single-mode oscillation at the current at the ordinal operations, with the adjustment of the degree of absorption.
Specifically, the semiconductor laser is formed so that the rate of change or the oscillation mode is obtained by setting the stripe width of the current block layer to a width of 4 to 6 xcexcm; or by setting the composition of Al of the above clad layers made of AlxGa1xe2x88x92xAs (0.4xe2x89xa6xxe2x89xa60.7) to not more than 0.65, current block layer being made of AlyGa1xe2x88x92yAs (0.5xe2x89xa6yxe2x89xa60.8), as well as setting the number of the well layers in the active layer to 5 or more layers; or more preferably forming the clad layer between the block layer and the active layer to a thickness of 0.03 to 0.1 xcexcm and forming the current block layer to a thickness of 0.2 to 0.5 xcexcm; or by inserting an absorption layer into the current block layer made of AyGa1xe2x88x92yAs (0.5xe2x89xa6yxe2x89xa60.8), the absorption layer being made of AlzGa1xe2x88x92zAs (0.08xe2x89xa6zxe2x89xa60.15) within the scope of the thickness t of 200 xc3x85xe2x89xa6txe2x89xa6500 xc3x85 in a distance h within the scope of 500 xc3x85xe2x89xa6hxe2x89xa61000 xc3x85 from the bottom of the current block layer, the clad layer being made of AlxGa1xe2x88x92xAs (0.4xe2x89xa6xxe2x89xa60.7); or by setting the composition of Al of the current block layer made of AlyGa1xe2x88x92yAs (0.5 less than y less than 0.8) into 0.65 or less, and forming the clad layer made of AlxGa1xe2x88x92xAs (0.4xe2x89xa6xxe2x89xa60.7) between the current block layer and the active layer to a thickness of 0.15 to 0.25 xcexcm; or by forming the clad layer between the current block layer and the active layer to a thickness of 0.25 to 0.5 xcexcm, the clad layer being made of AlxGa1xe2x88x92xAs (0.4xe2x89xa6xxe2x89xa60.7), and the current block layer being made of GaAs.